Nonaqueous secondary battery with lithium titanium cathode

ABSTRACT

A method for manufacturing a lithium secondary cell comprising a positive electrode containing a lithium titanate as an active material, a negative electrode containing a carbonaceous material as an active material, and an electrolytic solution comprising a solution of a lithium salt in an organic solvent. The lithium titanate preferably has a composition of the formula: 
     Li x Ti y O 4   
     (0.8≦x≦1.4 and 1.6≦y≦2.2).  
     The lithium secondary cell has a high capacity suitable for use as a power source for a wristwatch and good charge-discharge properties, at a nominal voltage of 1.5 V.

[0001] This application is the national phase under 35 U.S.C. §371 ofprior PCT International Application No. PCT/JP97/02008 which has anInternational filing date of Jun. 11, 1997 which designated the UnitedStates of America, the entire contents of which are hereby incorporatedby reference.

FIELD OF THE INVENTION

[0002] The present invention relates to a lithium secondary cell. Inparticular, the present invention pertains to lithium secondary cellssuitable as backup batteries for watches, power sources for portabledevices such as pagers, timers, etc., backup batteries for memories, andthe like.

PRIOR ART

[0003] Lithium secondary cells comprising a negative electrode made ofmetal lithium or a lithium alloy have been mainly developed as secondarycells. However, when the metal lithium or lithium alloy is contained inthe negative electrode, lithium ions in an electrolyte tend toprecipitate in the form of metal lithium on the negative electrodeduring recharging. The deposited lithium forms minute particles or growslithium dendrites on the surface of the negative electrode, and causes ashort-circuit in the cell. Thus, the charge-discharge cycle life of thecell is shortened. Consequently, lithium cells, which use neither metallithium nor a lithium alloy in the negative electrode and have a highenergy density, have been studied.

[0004] Currently, primary cells such as silver oxide cells are used aspower sources for wristwatches. However, the primary cells suffer fromproblems associated with disposal of the used cells. Thus, wristwatcheshaving built-in power generators, which require no replacement of thecells, have been developed, and electric double layer capacitors areused as power sources used in such wristwatches. However, the electricdouble layer capacitors have a small capacity per unit volume, and thusit is desired to develop substitute power sources for the electricdouble layer capacitor.

SUMMARY OF THE INVENTION

[0005] One object of the present invention is to provide a lithiumsecondary cell which can be charged and discharged, does not suffer fromthe problem of disposal, and has a high capacity so that it is suitableas a power source for a wristwatch.

[0006] Accordingly, the present invention provides a lithium secondarycell comprising a positive electrode containing lithium titanate as anactive material, a negative electrode containing a carbonaceous materialas an active material, and an electrolytic solution comprising asolution of a lithium salt in an organic solvent.

[0007] Since a lithium titanate as a positive electrode active materialand a carbonaceous material as a negative electrode active material areused in combination, the lithium ions can be easily doped and dedoped ata nominal voltage of 1.5 V, and thus a lithium secondary cell having ahigh capacity and good charge-discharge cycle properties is obtained.

BRIEF DESCRIPTION OF THE INVENTION

[0008]FIG. 1 is a cross section of an example of a lithium secondarycell according to the present invention.

[0009]FIG. 2 is a graph showing the discharge properties of the cells ofExamples 1 and 2 and comparative Example 1 in the first discharge.

[0010]FIG. 3 is a graph showing the charge-discharge cycle properties ofthe cells of Examples 1 and 2 and comparative Example 1.

DETAILED DESCRIPTION OF THE INVENTION

[0011] A lithium titanate used as a positive electrode active materialaccording to the present invention can be prepared by heating titaniumoxide and a lithium compound at a temperature of between 760 and 1100°C.

[0012] In general, the lithium titanate is represented by the formula(1):

Li_(x)Ti_(y)O₄  (1)

[0013] Usually, x and y in the above formula are numerals in the rangebetween 0.8 and 1.4 (0.8≦x≦1.4), and between 1.6 and 2.2 (1.6≦y≦2.2),respectively. In particular, the lithium titanate of the formula (1) inwhich x is 1.33 and y is 1.67 is preferable.

[0014] Titanium oxide may be either anatase or rutile. The lithiumcompound may be lithium hydroxide, lithium carbonate, lithium oxide, andthe like.

[0015] A positive electrode is preferably prepared by mixing lithiumtitanate, a conducting aid and a binder to obtain a positive electrodemixture, and shaping the mixture under pressure.

[0016] Examples of the conducting aid are scaly graphite, acetyleneblack, carbon black, and the like. Fluororesins are preferably used asbinders. Examples of fluororesins are polytetrafluoroethylene,polyvinylidene fluoride, and the like.

[0017] Proportions of the components constituting the positive electrodeare preferably 70 to 90 wt. % of the lithium titanate as the positiveelectrode active material, 5 to 20 wt. % of the conducting aid, and 1 to10 wt. % of the binder.

[0018] When the amount of the lithium titanate is less than the abovelower limit, the capacity of the cell tends to decrease and highcapacity may not be achieved. When the amount of the lithium titanateexceeds the above upper limit, the amounts of the electrical conductingaid and binder decrease correspondingly, and the conductivity orstrength of the positive electrode mixture may decrease.

[0019] When the amount of the conducting aid is less than the abovelower limit, the electrical conductivity may decrease. When the amountof the conducting aid exceeds the above upper limit, the amount of thelithium titanate decreases correspondingly and the capacity of the cellmay decrease.

[0020] When the amount of the binder is less than the above lower limit,the integrity of the positive electrode mixture may decrease and shapingof the mixture may become difficult. When the amount of the binderexceeds the above upper limit, the amount of the lithium titanatedecreases correspondingly and the capacity of the cell may decrease.

[0021] The production method for the positive electrode is not limitedto the above method, and compositions of the components are not limitedto the above described one.

[0022] A negative electrode is preferably prepared by mixing acarbonaceous material as a negative electrode active material and abinder to obtain a negative electrode mixture, and shaping the mixtureunder pressure.

[0023] Examples of the carbonaceous material as the negative electrodeactive material are synthetic graphite, natural graphite, lowcrystalline carbon, coke, anthracite (hard coal), and the like. Inparticular, synthetic graphite is preferable, since it can achieve thehigher capacity than other carbonaceous materials.

[0024] Fluororesins are preferably used as binders. Examples offluororesins are polytetrafluoroethylene, polyvinylidene fluoride, andthe like.

[0025] The proportions of the components constituting the negativeelectrode are preferably 80 to 95 wt. % of the carbonaceous material asthe negative electrode active material and 5 to 20 wt. % of the binder.

[0026] When the amount of the carbonaceous material as the negativeelectrode active material is less than the above lower limit, it may bedifficult to obtain a lithium secondary cell having a high capacity.When the amount of the carbonaceous material exceeds the above upperlimit, the amount of the binder decreases, and thus the integrity of thepositive electrode mixture may decrease and shaping of the mixture maybecome difficult.

[0027] The production method for the negative electrode is not limitedto the above method, and compositions of the components are not limitedto the above described one. For example, a conducting aid may be addedto the negative electrode mixture.

[0028] In the present invention, the cell comprises an electrolyticsolution which is prepared by dissolving a lithium salt in an organicsolvent. Examples of the organic solvent used as the solvent for theelectrolytic solution are propylene carbonate, ethylene carbonate,butylene carbonate, γ-butyrolactone, 1,2-dimethoxyethane,dimethoxymethane, tetrahydrofuran, dioxolane, and the like.

[0029] Examples of the lithium salt are LiN(CF₃SO₂)₂, LiClO₄, LiPF₆,LiBF₄, LiAsF₆, LiSbF₆, LiCF₃SO₃, LiCF₃CO₂, LiC_(n)F_(2n+1)SO₃ (n≧2),LiN(CF₃CF₂SO₂)₂, and the like. Among them, LiN(CF₃SO₂)₂, LiPF₆, LiCF₃SO₃and LiBF₄ are preferably used since they have high conductivity, and arethermally stable.

[0030] The concentration of the lithium salt in the electrolyticsolution is not limited, and is usually between 0.1 and 2 mole/lpreferably between 0.4 and 1.4 mole/l.

[0031] The structure and production method of the lithium secondary cellof the present invention are substantially the same as those ofconventional lithium secondary cells except that the above positive andnegative electrodes and electrolytic solution are used.

EXAMPLES

[0032] The present invention will be illustrated by the followingExamples, which do not limit the scope of the present invention in anyway.

Example 1

[0033] Anatase titanium oxide (2 moles) and lithium hydroxide (1 mole)were mixed and calcined using an electric furnace in air at 800° C. for8 hours, and a lithium titanate was obtained. The composition of thislithium titanate was analyzed by atomic absorption analysis, and foundto be Li_(1.33)Ti_(1.67)O_(4.)

[0034] The obtained lithium titanate (100 wt. parts), carbon black (5wt. parts) and graphite (5 wt. parts) as conducting aids, andpolytetrafluoroethylene (5 wt. parts) as a binder were mixed inisopropanol to prepare a positive electrode mixture. After evaporatingoff the solvent, the positive electrode mixture was molded in the formof a pellet having a diameter of 6.0 mm and a thickness of 0.5 mm. Thepellet was dried and dehydrated with a far-infrared drier at 250° C. for30 minutes to form a positive electrode.

[0035] Separately, synthetic graphite (90 wt. parts) and polyvinylidenefluoride (10 wt. parts) as a binder were mixed in N-methylpyrrolidone toprepare a negative electrode mixture. After evaporating off the solvent,the negative electrode mixture was molded in the form of a pellet havinga diameter of 3.5 mm and a thickness of 1.0 mm. The pellet was dried anddehydrated with a far-infrared drier at 120° C. for 30 minutes to form anegative electrode.

[0036] An electrolytic solution, which was prepared by dissolvingLiN(CF₃SO₂)₂ in a mixed solvent of ethylene carbonate and diethylcarbonate in a volume ratio of 1:1 at a concentration of 1.0 mole/l, wasused.

[0037] Using the above positive and negative electrodes and electrolyticsolution, a lithium secondary cell having the structure shown in FIG. 1and an outer diameter of 6.7 mm and a height of 2.1 mm was assembled.

[0038] In FIG. 1, a positive electrode 1 consisted of a press moldedarticle of a positive electrode mixture containing the lithium titanate(Li_(1.33)Ti_(1.67)O₄) as an active material, carbon black and graphiteas conducting aids, and polytetrafluoroethylene as a binder.

[0039] A negative electrode 2 consisted of a press molded article of anegative electrode mixture containing synthetic graphite as an activematerial and polyvinylidene fluoride as a binder.

[0040] A separator 3 made of polypropylene non-woven fabric was insertedbetween the positive electrode 1 and negative electrode 2.

[0041] During assembling of the cell, the negative electrode 2 was dopedwith lithium ions in the presence of an electrolytic solution whileplacing metal lithium in an amount corresponding to 80% of the electriccapacity of the positive electrode on the opposite side of the separator3.

[0042] The positive electrode 1, negative electrode 2, separator 3 andelectrolytic solution were sealed in a space formed by a positiveelectrode can 4 made of stainless steel, a negative electrode can 5 madeof stainless steel, and an insulation packing 6 made of polypropylene.

Example 2

[0043] A lithium secondary cell was produced in the same manner as inExample 1 except that an electrolytic solution, which had been preparedby dissolving LiPF₆ in place of LiN(CF₃SO₂)₂ in a mixed solvent ofethylene carbonate and diethyl carbonate in a volume ratio of 1:1 at aconcentration of 1.0 mole/1, was used.

Comparative Example 1

[0044] A lithium secondary cell was produced in the same manner as inExample 1 except that lithium iron oxide (LiFe₅O₈) was used as apositive electrode active material in place of a lithium titanate.

[0045] Each of the cells produced in Examples 1 and 2 and ComparativeExample 1 was charged and discharged under the following conditions, andthe discharge property in the first discharge, and the charge-dischargecycle property were evaluated:

[0046] Charging conditions: constant current, 0.1 mA and charge cutvoltage, 2.4 mA

[0047] Discharging conditions: constant current, 0.1 mA and dischargecut voltage, 0.4 mA

[0048] The discharge properties in the first discharge are shown in FIG.2, and the charge-discharge properties are shown in FIG. 3.

[0049] As seen from FIG. 2, the cells of Examples 1 and 2 had betterflatness of the cell voltage around 1.5 V and larger cell capacity downto 0.4 V than the cell of Comparative Example 1. Thus, the cells ofExamples 1 and 2 had a high capacity.

[0050] As seen from FIG. 3, the cells of Examples 1 and 2 had the largercell capacity than the cell of Comparative Example 1 after the samenumber of cycles. Furthermore, the former cells suffered less decreaseof cell capacity due to the increase of the number of cycles than thelatter cell. That is, the cells of Examples 1 and 2 had goodcharge-discharge cycle properties.

[0051] In contrast, the cell of Comparative Example 1 had a small cellcapacity, and the cell capacity dropped sharply in the early cycles ofcharge and discharge. That is, this cell had low charge-discharge cycleproperties. These properties of the cell of Comparative Example 1 may beattributed to the destabilization of the crystal structure of lithiumiron oxide, which was used as the positive electrode active material,during the charge and charge cycles.

[0052] A lithium secondary cell was produced in the same manner as inExample 1 or 2 using the same electrolytic solution and negativeelectrode, and the lithium titanate having the composition of Li₁Ti₂O₄or Li_(0.8)Ti_(2.2)O₄ in place of Li_(1.33)Ti_(1.67)O₄, and a cellcapacity and charge-discharge cycle properties were evaluated. Theresults were the same as those in Examples 1 and 2.

[0053] As explained above, the present invention can provide lithiumsecondary cells having a high capacity and good charge-discharge cycleproperties at a nominal voltage of 1.5 V, since a lithium titanate ofthe formula: Li_(x)Ti_(y)O₄ is used as a positive electrode activematerial, and a carbonaceous material such as synthetic graphite is usedas a negative electrode active material.

What is claimed is:
 1. A method of manufacturing a lithium secondarycell which comprises: assembling a positive electrode, a negativeelectrode, and a separator between the positive electrode and thenegative electrode, in an electrolytic solution, the positive electrodecontaining lithium titanate as an active material, the negativeelectrode containing carbonaceous material as an active material, andthe electrolytic solution comprising a lithium salt in an organicsolvent; doping lithium ions into the negative electrode; and sealingthe positive electrode, the negative electrode, the separator and theelectrolytic solution in a space formed by a positive electrode can, anegative electrode can and an insulation packing.
 2. The method of claim1, wherein the doping is performed by placing metallic lithium in thecell.
 3. The method of claim 1, wherein the doping is performed byplacing in the cell metallic lithium in an amount corresponding to about80% of an electric capacity of the positive electrode.
 4. The method ofclaim 1, wherein the lithium titanate has a composition represented bythe formula: Li_(x)Ti_(y)O₄ wherein x and y are between about 0.8 andabout 1.4 (0.8≦x≦1.4) and about 1.6 and about 2.2 (1.6≦y<2.2),respectively.
 5. The method of claim 1, wherein the lithium ions aredoped and dedoped at a nominal voltage of about 1.5 V.
 6. The method ofclaim 1, wherein the lithium titanate was prepared by heating titaniumoxide and a lithium compound at a temperature between about 760 andabout 1100° C.
 7. The method of claim 1, wherein the positive electrodeis prepared by: mixing the lithium titanate, a conducting aid and abinder; and shaping under pressure.
 8. The method of claim 1, whereinthe negative electrode is prepared by: mixing the carbonaceous materialand a binder to obtain an negative electrode mixture; and shaping thenegative electrode mixture under pressure.
 9. The method of claim 1,wherein the electrolytic solution is prepared by: dissolving a lithiumsalt in an organic solvent.
 10. The method of claim 9, wherein theorganic solvent is at least one selected from the group consisting ofpropylene carbonate, ethylene carbonate, butylene carbonate,γ-butyrolactone, 1,2-dimethoxyethane, dimethoxymethane, tetrahydrofuranand dioxolane.
 11. The method of claim 9, wherein the lithium salt is atleast one selected from the group consisting of LiN (CF₃SO₂)₂, LiClO₄,LiPF₆, LiBF₄, LiAsF₆, LiSbF₆, LiCF₃SO₃, LiCF₃CO₂, LiC_(n)F_(2n+1)SO₃ andLiN(CF₃CF₂SO₂)₂.
 12. The method of claim 1, wherein a concentration ofthe lithium salt in the electrolytic solution is between about 0.1 andabout 2 mole/l.
 13. The method of claim 6, wherein the titanium oxide isanatase or rutile.
 14. The method of claim 6, wherein the lithiumcompound is at least one selected from the group consisting of lithiumhydroxide, lithium carbonate and lithium oxide.
 15. The method of claim1, wherein the carbonaceous material is at least one selected from thegroup consisting of synthetic graphite, natural graphite, lowcrystalline carbon, coke and anthracite.
 16. The method of claim 7,wherein the binder is a fluororesin.
 17. The method of claim 8, whereinthe binder is a fluororesin.
 18. The method of claim 7, wherein thepositive electrode contains about 70-90 wt % of the lithium titanate,about 5-20 Wt % of the conducting aid, and about 1-10% of the binder.19. The method of claim 7, wherein the conducting aid is at least oneselected from the group consisting of scaly graphite, acetylene blackand carbon black.
 20. The method of claim 8, wherein the negativeelectrode contains about 80-95% of the carbonaceous material and about5-20% of the binder.